CHAP. II. FUNCTION OF NUTRITION. 179 



of sap or " lymph." Essentially, this sap is nearly pure 

 water ; but in order that it may become effective in 

 nourishing the plant, it must contain carbonic acid, or 

 at least some carbonaceous material capable of being con- 

 verted into carbonic acid by a subsequent process, which 

 we shah 1 presently describe. In Dicotyledonous woody 

 stems, it has been clearly ascertained that the course of 

 the sap is up the woody fibre, and especially through the 

 alburnum, but that it does not ascend in any appreciable 

 quantify through the pith or bark. It is then carried 

 onward through the branches and into the leaves. In 

 the internal parts of old trunks, the sap accumulates in 

 large quantities about the spring of the year, and is there 

 retained under a certain degree of compression ; for if 

 the tree be felled at this season, it flows most readily 

 from those central parts which have ceased to possess 

 any vitality, and sometimes it even issues in a jet during 

 a few seconds, when the trunk is first severed. Whether 

 or not any distinct modification takes place whilst the 

 sap is moving onward, analogous to the effects of diges- 

 tion in animals, has not been clearly ascertained. It is 

 certain, indeed, that if a tree is tapped at different 

 heights, when the sap is rising with the greatest energy, 

 the liquid obtained from the lower parts of the stem is 

 purer than that which is derived from the upper parts. 

 But this may be ascribed to the complete admixture 

 which takes place between the juices previously elabo- 

 rated and the ascending sap. which thus becomes thick- 

 ened by them as it moves onward. 



(164.) Channels for the Sap. Some authors suppose 

 the sap to be propelled through the vascular system, 

 whilst others consider it to rise through the intercellular 

 passages, and others again imagine that it passes from 

 cell to cell, through the elementary membrane of 

 which they are formed. The great difficulty in de- 

 termining the precise channel through which the pro- 

 gression of the sap takes place, must be ascribed to the 

 perfect transparency of the vegetable membrane, and the 

 extreme minuteness of these organs themselves. By 

 N 2 



